Immunization with small amounts of inactivated Moloney leukemia virus can increase tumorigenesis by Moloney sarcoma virus. Immunization is a potential mode of immunotherapy and of prevention of cancer; therefore, it is necessary to determine whether increased tumorigenesis after immunization is a general phenomenon. The purpose of this study is twofold: (1) to determine whether increased tumorigenesis after immunization is a general phenomenon in virus-induced primary neoplasia and (2) to define some of the immunologic parameters of the stimulation, particularly its immunologic specificity and its mediator(s). The information obtained from this study will be used to evaluate whether stimulation is a potential hazard of immunization and, further, will aid in the analysis of the mechanism of stimulation. More knowledge about this mechanism will help in deciding whether or not the stimulatory effect of immunization can be avoided.